The invention relates to the field of mobile telephone systems. A subscriber of a mobile telephone system, that is, a subscriber station, e.g. a mobile telephone or a mobile station may be connected to a mobile network or system, whereby it is connected to the mobile network via system channels maintained by the base stations of the mobile network.
In addition to the system channels located in mobile telephone systems, so-called direct mode channels may be used in association with the mobile system, i.e. communication takes place in a direct mode. Subscriber stations communicating in the direct mode do not communicate directly with the mobile network or the base stations related to it. Direct mode channels are frequencies on which mobile telephones or other means of communication can communicate without the system directly with each other, or via repeater stations, either with the base stations of the system or with other mobile stations.
Direct mode channels are typically used in situations in which e.g. a group of hand-held mobile phones communicate with each other so remote from the base stations that the use of system channels is not possible.
Another important use of direct mode channels is increasing the capacity when the traffic in the system rapidly increases (incident) in some part of the service area of the system, e.g. in some spot-like part.
A direct mode channel is a channel that is typically not used by the system. It may be e.g. a channel whose channel spacing is the same as that in the mobile communications system, e.g. 12.5 kHz or 25 kHz. On direct mode channels a mode of simplex operation is used. From the group of the mobile stations operating on the direct mode channel, the transmitting party has tuned his transmitter onto the channel and is transmitting speech or data information. The other mobile stations that have been activated onto the direct mode channel have tuned their receivers onto the same frequency so that they will directly hear the transmission.
On a direct mode channel, communication may take place by means of analog modulation or digital modulation. A mobile station transmitting on the channel may also use signalling information, such as information on access rights and priorities, or the group communicating on the channel, or data communication. On a direct mode channel encryption may be carried out, or plain speech can be transmitted.
Subscriber stations operating in the direct mode communicate with other subscriber stations on a direct mode channel without having a direct connection to the base stations of the mobile network. Subscriber stations operating in the direct mode, however, may communicate with the mobile network via repeater stations. A repeater station typically comprises two transceivers connected to each other.
The prior art solutions are attended by the problem how to make a mobile station operating in accordance with the dual watch facility save its own battery when it is communicating both on a direct mode channel and on the system channel of the mobile communication system. In communication carried out in accordance with the standard dual watch facility, the mobile station communicating on the direct mode channel must listen to every second time-slot. This strains the batteries of the mobile station and causes a heavy load to the mobile station in other respects, as well. On the other hand, if the mobile station did not listen to the control channel of the system at all, the messages addressed to the mobile station from the system would not be received, and the system would not know when the mobile station is possibly listening to the control channel.
Another problem related to prior art solutions is the fact that in a situation in which a mobile station is transmitting on a direct mode channel, it is not possible for the mobile station to operate in accordance with the dual watch facility in certain frames on the control channel of the system, since it must simultaneously receive a pre-emption frame on the direct mode channel, in which frame the other mobile stations communicating on the direct mode channel can request a speech turn/call transfer for themselves. The system is aware of a mobile station transmitting on a traffic channel subject to the system, and therefore it is possible for the system to buffer the message addressed to this mobile station until the end of the speech turn when necessary. The system has no information available on traffic taking place on the direct mode channel, and it is thus not aware of the identifier of the mobile station that is transmitting currently, and it thus cannot buffer messages addressed to it.